Banner featured-Mythic math
Per Gamepur: 50% chance the rolled Mythic is the main featured, 20% is secondary featured, 1% is non-featured. Bluma has ~0.25% chance if main featured.
Soft pity ramp
Standard banner: 400-pity tier. Hard pity is community folklore (~10,000 summons per TechWiser) and not officially published.
Immortal vs Secret confusion
Immortal is a TRAIT reroll outcome at ~0.1% with ~1,300 pity. Secret units are banner summons with community 0.01-0.1% rates (sources disagree).
Honest gap
No officially published per-unit Secret summon rate. Live banner screen is the authority.